Coaxial cables are typically manufactured by forming, over a center wire, a molded dielectric, and forming a braided outer conductor over the dielectric. Alternatively it may be manufactured by inserting a center wire conductor within a hollow braided wire (or some other flexible metal tubing) with spacers. The dielectric spacers or shielding that run down the length of the cable prevents electrical contact of the center conductor to the outer conductor and holds a pre-defined distance between the center conductor and the outer conductor. The electrical terminals of the center conductor and the outer conductor are typically connected to a source generating microwave signals or power in order to transmit electrical signals or energy effectively through the cable.
In such cables the center conductor is usually a solid wire. Cables with hollow conductors have been disclosed, for example, in Guilbert et al, U.S. Pat. No. 5,006,825 (1991), and Ditscheid et al, U.S. Pat. No. 3,600,709, (1971). However, the hollow core coaxial cables described in these patents tend to be large, inflexible, and expensive, and do not lend themselves easily to miniaturization. For applications where a very small diameter cable is required, the existing techniques do not scale down well in size, for example, to cable diameters below 2 mm, 1 mm, or 0.3 mm.
In addition, hollow glass waveguides for transmitting mid-infrared wavelengths of between 2.5-25 μm have been constructed based on hollow glass fibers having inner diameters of between 250-1000 μm with a conductor such as silver coated on the inside wall of the fiber at thicknesses from 0.2-0.4 μm. The conductor is then coated with thin layers of dielectric materials such as polyimides, aluminum oxide, titanium dioxide, silicon dioxide, zinc chalcogenides (oxides, sulfides, selenides, and tellurides) silicon nitride, compound semiconductors and various metal halogen compounds, including silver chloride, silver bromide and silver iodide. For example, Matsuura et al., in “Optical properties of small-bore hollow glass waveguides,” Appl. Opt. 34, 6842 (1995), describes the formation of silver coatings onto the inside of glass capillaries having inner diameters as small as 250 μm and subsequently coating the silver layer with silver iodide. Croitoriu et al, U.S. Pat. No. 4,930,863 (1990) discloses essentially the same structure but larger and in tubes constructed of various polymers, including polyethylene, polytetrafluoroethylene, fluorinated ethylene-propylene, perfluoroalkoxy olefin as well as polyethylene, polypropylene, nylon 6, nylon 11, silicone rubber, rubber, polyvinyl chloride and polystyrene. However, these disclosed waveguides are hollow and do not have a central conductor with a metalized outer conductor surface separated by a dielectric and are therefore not coaxial nor in TEM (transverse electromagnetic mode). Hollow core waveguide techniques are limited in their size as a function of the frequency of interest and at microwave frequencies that would require core diameters to range from many centimeters across to millimeters across in the 10-100 GHz range unlike a TEM transmission line such as coaxial cables. In addition they are limited in bandwidth to a particular frequency band based on the diameter of the waveguide.
Therefore, there remains a need for a flexible coaxial cable and waveguide that is inexpensive and can be scaled down in size below the dimensions of hollow core waveguides, that can be easily fabricated, and that can transmit RF, microwaves or millimeter waves. Moreover, there remains a need for a flexible coaxial cable and wave guide that is made inexpensively, with tight mechanical tolerances, and in which the conductor on the inner and outer surfaces are optionally removed to form a pattern or deposited to form a pattern leaving one or more clear areas. Such patterns can form antenna elements, inductors and other microwave components as well as fashion a port to allow entry and/or egress of high frequency signals through the dielectric without breaching it. Moreover, there remains a need for a flexible coaxial cable and wave guide that can be formed into long lengths and cut into precise short lengths and which can be bundled in various ways, including flexible bundled cables, flexible ribbons and the like, and for which existing manufacturing and cabling and connectorization infrastructure is largely capable of producing with high precision and low cost.